The method described above is a known technique for coating substrate with (thin) layers of coating material; the method is usually referred to as physical vapour deposition (PVD). This technique is in widespread use in the electronics and optical industries, in the glass industry and in the manufacture of metal-coated plastic sheets for all kinds of applications. PVD is an attractive coating method because the quality which can be achieved is high and there are no waste products produced.
When using PVD, the coating material firstly has to be converted to the vapour phase. This is achieved by heating the coating material in a chamber in which there is a very low background pressure, known as a vacuum chamber. As a result of the heating, the coating material changes to a vapour until a pressure which is in thermodynamic equilibrium with the hot surface of the coating material where the vapour is formed is reached. This equilibrium vapour pressure is the most important parameter for the transfer rate of the coating material to the substrate on which the vapour is deposited.
The equilibrium vapour pressure is dependent on the temperature of the coating material. To achieve a reasonable transfer rate of coating material to the substrate, i.e., a reasonable quantity of coating material which is deposited on the substrate per unit time, the coating material generally has to be heated to high temperatures. These temperatures are often of the order of half the boiling point at atmospheric pressure or sometimes even higher. In practice, the temperatures for metals are between approximately 600° C. for zinc and approximately 2200° C. for niobium and rhenium.
Metals such as tantalum, molybdenum and tungsten require such high temperatures that they are not used for PVD. Metals such as titanium, chromium, nickel, aluminium and the like are rarely used as the material transfer rates are low.
A drawback of using PVD is that the transfer rates are limited primarily by the fact that the coating materials which have to be vaporized are always in the liquid state on account of the high process temperatures. Consequently, the material has to be in a crucible, which may be made, for example, from a ceramic material or from copper. In the latter case, intensive cooling with water is required, so that a thin film of solidified coating material covers the copper, with the result that the copper is prevented from melting or being vaporized as well and the copper is not affected. One disadvantageous consequence of cooling of a copper crucible is that a significant proportion of the heat supplied is lost as a result of the cooling. The use of a ceramic crucible is limited to coating materials which do not enter into a chemical reaction with the crucible material at the high process temperatures. The supply of the thermal energy required also presents a problem when using a ceramic crucible, since most ceramic materials are poor heat conductors. JP 08-104981 to Teruyuki discloses a PVD device employing a crucible and is incorporated herein by reference in its entirety.